1. Field of the Invention
This invention relates to a method and apparatus for locating defective fuel rods which are combined into fuel elements and are used in water cooled reactor cores.
2. Description of the Prior Art
The core of a water cooled reactor consists of approximately 40 - 50 thousand fuel rods which are combined to form groups of approximately 200 pieces known as fuel elements. A fuel element consists of two end pieces, the guide tubes for housing the the control or positioning rods, the spacing devices for positioning the fuel rods, and the fuel rods. The fuel rods, whose covering is made of Zircaloy-4, contain the core fuel in the form of oxides and are closed on both ends by welded end caps.
In operation, localized sealing problems or looseness occurs in individual rods. This may cause a discharge of fission products in gas form into the flow of coolant medium and thereby increase its radioactivity.
To a certain extent, fission products in the cooling system can be tolerated. However, it is desirable to keep their amount low because of the radiation density in the vicinity of the reactor cooling system.
Because of this problem, the fuel elements are routinely tested during each fuel element change by a known "Sipping" test. In this test, the fuel element in the fuel element storage basin is placed under water in a container. The fuel rods and the water in the container heat up due to the heat from radioactive decay. If this fuel element contains defective rods, radioactive fission products are discharged by the defective rods during the heating and get into the water. By withdrawal of water samples and measurement of their radioactivity, it can be determined if the element contains defective rods. This procedure is an integral method, in that it only determines if a fuel element contains any defective rods; however, it does not give information as to their position. In order to restore a fuel element with defective rods to service, it is necessary to locate the defective rods in the fuel element, thereafter to pull them out and to fill the open positions with new rods or dummy rods. In the known procedure for locating defective fuel rods, all rods of a fuel element are successively completely or partially pulled and examined for defects by means of turbulent eddy currents or ultrasonic tests. The rods which are free of defects, are again inserted into the fuel element and the defective rods are replaced. This procedure is very time-consuming and complicated. A further disadvantage is that all rods have to be removed or partially removed from the fuel element for testing. During pulling and re-insertion of the rods, the spacers cause scratches and grooves on the covering tubes which can be the starting point for later fuel rod defects.
In addition, as described in German Patent No. 2 314 650, another method is known wherein the rod cover tube is heated beneath the end cap by high frequency energy. The water within the heating zone steams off and the presence of water inside of the fuel rod can be deducted from the temperature behavior at the end cap.
This method has the disadvantage that the fuel rods can only be tested individually or in small groups. Considerable difficulties are thereby encountered in order to manipulate the heating coil over the 11 mm diameter fuel rod through a water layer of several meters in thickness.
It has also been shown to be a disadvantage that the temperature-time cycle of each fuel rod must be measured individually which results in a time period of several minutes per rod. A considerable time will pass until all fuel rods of a fuel element are tested, so that it is difficult to test all fuel elements which contain defective fuel rods during a fuel element replacement cycle.